Add direct messages (#52)

A DM is a Room with a new is_dm flag, not a separate model -- reuses
all the membership/message/WS plumbing Room already has instead of
duplicating it. The room's `name` (still required + globally unique)
is an internal, never-displayed token derived deterministically from
the two participants' sorted user IDs (dm_room_name), which makes
find-or-create a single indexed lookup and gets free race-condition
safety from the existing unique constraint -- a concurrent double-
start from both people just hits the same IntegrityError->retry-as-
lookup path create_room already established.

Both participants get the plain 'member' role (no owner/admin
distinction makes sense for a 1:1 DM), which incidentally reuses
every existing role gate to block add-member, room-settings edits,
and join-via-browse on a DM for free. update_room also gets an
explicit is_dm guard independent of that, since renaming a DM isn't
just a privacy concern -- it would silently corrupt the find-or-create
invariant. DMs are excluded from both Browse Rooms and the admin
portal's room listing (fully private, per scope).

GET /api/rooms/mine precomputes each DM's other participant (name,
avatar, presence) as dm_partner in one batched query, so the sidebar
can render a DM row without a fetch per row. Frontend: a new "Direct
Messages" sidebar section (searchable by partner name, not the
internal room name), clicking someone in the People list starts or
resumes a DM, and the chat header/composer/RoomInfoPanel all render
the partner's identity instead of a room name where it's a DM.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/models/room.py

@@ -15,6 +15,10 @@ class Room(Base):
     description: Mapped[str | None] = mapped_column(Text)
     is_private: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     is_archived: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
+    # #52: a DM is a Room whose `name` is an internal, never-displayed
+    # deterministic token (see room_service.dm_room_name) rather than a
+    # user-chosen name -- see that function's docstring for the scheme.
+    is_dm: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False)
     owner_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"), nullable=False)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), nullable=False